The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method of screening disc drives for various frequencies of resonance.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of disc drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator. The voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor which is used to rotate the actuator and the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. A yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
Two of the ever constant goals of disc drive designers is to increase the data storage capacity of the disc drive and to decrease the amount of time needed to access data. Decreasing the amount of time needed to access data can also be thought of as increasing the speed at which data can be retrieved. Increasing the speed at which data can be retrieved is very desirable in a disc drive. The decrease in access time increases the speed at which a computer system can perform operations on data. When a computer is commanded to perform an operation on data or information that needs to be retrieved, the time necessary to retrieve the data from the disc is generally the bottleneck in the operation. When data is accessed more quickly, more transactions can generally be handled by a computer in a particular unit of time.
In order to achieve a faster servo response, the gain of the servo system is very often raised. It will have an impact of raising the mechanical structural resonance peak at high frequency and the drive will be more susceptible to resonance. It is important to minimize resonance in order to improve disc drive""s through-put performance. If the actuator arm does not resonate at frequencies associated with the normal operation of the drive, track following will also be improved. Track following is the ability of the disc drive and the servo system to keep the transducer for reading and writing positioned over a desired track. If resonances in the actuator arm are eliminated or minimized, track following is more achievable since the servo system will not be attempting to counter the effects of a resonating arm swinging across a desired track from an off track position on one side to an off track position on the other side of the track.
In order to track follow in a disc drive with increased TPI, the servo open loop bandwidth of the system must also be pushed or increased. This also increases the actuator""s susceptibility to vibration induced at the actuator""s resonant frequency, which may result in greater off-track disturbances of the read/write transducer. The situation becomes worse as mechanical structural damping and stiffness (resonant frequency) vary with temperature. At warmer temperatures such as the operating temperature of the disc drive, the amplitude of the resonant frequency may be raised. This will result in less gain margin of the servo loop.
A common approach to address the mechanical resonance problem is to include notch filters that attenuate the resonant modes at particular frequencies. A notch filter can be implemented in an analog hardware circuit notch or software notch. In either case, there are several problems associated with notch filters. Among the problems are that due to the different behavior of mechanical structural response at different temperature, the amplitude or even the frequency of resonant modes may change. Therefore, it is possible that one disc drive having acceptable margin at normal temperature may fail at a higher temperature. Furthermore, if the notch filter is implemented in an analog fashion, the thermal effects on the components will vary the center frequency of notch filter. Although the center frequency drifting problem does not exist for a digital notch filter, implementing a software digital notch will take up CPU time and affects the disc drive""s performance. This is a major concern in disc drives manufactured for desktop operations where there is emphasis on low cost. Yet another problem is that the introduction of a notch filter also will introduce phase margin loss. As a result, there will not be many notch filters for every potential resonant mode.
During manufacturing, disc drives found to have resonance in the actuator arms are rejected. Disc drives having resonance at certain frequencies are considered as having a fatal error and are scrapped. One problem is that current screening processes are slow and consume computer time. Generally, screening includes analyzing the position error signal using digital fourier transform (DFT) to analyze the frequency components of the position error signal. This requires computer time and may slow down the testing portion of the manufacturing process.
What is needed is a method and apparatus to quickly screen disc drives for various frequencies, including frequency around a calculated frequency. There is also a need for a screening test that will determine resonant frequencies for different situations, such as when a disc drive is at an operating temperature or when the mechanical components cause a shift in the frequency. There is also a need for screening test which is quick and which can be easily incorporated into the current manufacturing and testing process. There is still a further need for a screening test that will indicate the amount of gain margin for the resonance.
A disc drive includes a base and a disc rotatably attached to the base. The disc drive also includes an actuator assembly rotatably attached to said base and a device for moving the actuator assembly. The actuator assembly includes an actuator arm and a transducer head in a transducing relationship with respect to the disc. The transducer is attached to the actuator arm. A method for screening disc drives for resonant frequencies associated with the actuator arm includes the steps of following a track within the disc drive, and selectively boosting the servo control signal. The method further includes a step of monitoring a position error signal from a transducer attached to an actuator arm of a disc drive. The position error signal is monitored by filtering the position error signal using a bandpass filter. Boosting the servo control loop includes tuning the magnitude of the booster signal gain.
In one embodiment, the magnitude of the booster gain is tuned to approximately 5 dB. Monitoring includes inputting the position error signal to a bandpass filter, and checking the output of the bandpass filter for indications of frequency resonance having a value within a selected range of frequency values. The selected range of frequency values is near a calculated value for resonant frequency associated with the actuator assembly. The selected range of frequency values may also include shifts from the calculated value for resonant frequency corresponding to a shift in the resonant frequency of the actuator assembly due to thermal effects. The selected range of frequency values may also include shifts from the calculated value for resonant frequency corresponding to a shift in the resonant frequency of the actuator assembly due to mechanical differences in the parts making up the actuator arm assembly of the disc drive. If resonant frequencies are found within the selected range, the disc drive is rejected.
A disc drive device includes a base, a disc rotatably attached to the base, an actuator arm for carrying a transducer head in a transducing relation with respect to the disc, and a disc drive controller, coupled to the actuator arm. The disc drive controller further includes a servo controller, coupled to the actuator arm. The servo controller monitors a position error signal in order to follow a track on the disc drive. A band pass filter is used to monitor the position error signal for resonant frequencies in the actuator arm. The bandpass filter monitors the position error signal for resonant frequencies within a selected range of resonant frequencies. The disc drive further includes a position error signal switch associated with the position error signal. The position error signal switch is for inputting the position error signal to the bandpass filter. The disc drive device also includes a booster signal generator which can be added to the servo control signal. A switch which can be enabled to add the booster signal to the servo control signal. The switch can also be termed a booster signal. In one embodiment, the booster signal switch and the position error signal switch add the booster signal and the bandpass filter to the servo control loop.
Most generally, an apparatus for testing a disc drive includes a servo control loop a device for altering the control signal and monitoring the feedback signal of the servo control loop. Alternatively, a disc drive includes a base, an actuator arm rotatably attached to the base, at least one disc rotatably attached to the base, where the actuator arm carries a transducer head in a transducing relation with respect to the disc, and a device for screening the disc drive for selected resonant frequencies.
Advantageously, the inventive method and apparatus quickly screens disc drives for various frequencies, including frequency around a calculated frequency. The method and apparatus will also determine resonant frequencies for different situations, such as when a disc drive is at an operating temperature or when the mechanical components cause a shift in the frequency. The screening test indicates the amount of gain margin for the resonance frequencies found. The method and apparatus is quick and can be easily incorporated into the current manufacturing and testing process.